Optical receiver for multimode communications

Abstract

An optical receiver ( 20 ) for multimode communications comprises: a mode demultiplexer ( 21 ) having an input connected to a multimode link ( 22 ) and a plurality of output lines ( 231 - 234 ), wherein the mode demultiplexer is adapted to couple each of the modal components of the optical signal substantially into a selected one of the output lines, a plurality of coherent optical detectors ( 251 - 254 ) respectively connected to the output lines to produce a set of electrical digital signals each comprising an in-phase component and a quadrature-phase component, a plurality of independently adjustable optical delay devices ( 241 - 244 ) arranged on the output lines to impart a selected delay to each of the corresponding modal components, and a signal processing device ( 26 ) adapted to process the digital signals to recover the independent modulations of the respective modal components by inverting a mode-mixing characteristic of the multimode link.

The invention claimed is: 
     
       1. An optical receiver for multimode communications, comprising:
 a mode demultiplexer having an input and a plurality of output lines, the input being adapted to receive an optical signal having a plurality of modulated modal components, wherein each of the plurality of modulated modal components is coupled into one of the plurality of output lines; 
 a plurality of independently adjustable optical delay devices, wherein each of the independently adjustable optical delay devices is connected to one of the plurality of output lines; 
 a plurality of coherent optical detectors producing a plurality of electrical digital signals, wherein each of the plurality of coherent optical detectors is connected to one of the plurality of independently adjustable optical delay devices; and 
 a signal processing device adapted to process the plurality of electrical digital signals. 
 
     
     
       2. The optical receiver of  claim 1 , wherein at least one of the plurality of independently adjustable optical delay devices comprises a first delay device having a first plurality of optical fibers wherein each optical fiber has a different length and wherein one of the plurality of modulated modal components is substantially coupled into a selected one of the first plurality of optical fibers. 
     
     
       3. The optical receiver of  claim 2 , wherein the different lengths of the first plurality of optical fibers are successive integer multiples of a reference length. 
     
     
       4. The optical receiver of  claim 2 , wherein the first plurality of optical fibers are single mode optical fibers. 
     
     
       5. The optical receiver of  claim 2 , wherein at least one of the plurality of independently adjustable optical delay devices comprises a second delay device having a second plurality of optical fibers wherein each optical fiber has a different length and wherein one of the plurality of modulated modal components from the first plurality of optical fibers is coupled into a selected one of the second plurality of optical fibers. 
     
     
       6. The optical receiver of  claim 5 , wherein the different lengths of the second plurality of optical fibers are equal to the different lengths of the optical fibers in the first plurality of optical fibers times an integer multiplier. 
     
     
       7. The optical receiver of  claim 6 , wherein the integer multiplier is equal to a number of optical fibers in the first plurality of optical fibers. 
     
     
       8. The optical receiver of  claim 5 , wherein the first and second pluralities of optical fibers are single mode optical fibers. 
     
     
       9. An optical receiver for multimode communications, comprising:
 a mode demultiplexer having an input and a plurality of output lines, the input being adapted to receive an optical signal having a plurality of modulated modal components, wherein each of the plurality of modulated modal components is coupled into one of the plurality of output lines; 
 a plurality of independently adjustable optical delay devices, wherein each of the independently adjustable optical delay devices is connected to one of the plurality of output lines; 
 a plurality of coherent optical detectors producing a plurality of electrical digital signals, wherein each of the plurality of coherent optical detectors is connected to one of the plurality of independently adjustable optical delay devices; 
 a signal processing device adapted to process the plurality of electrical digital signals; and 
 a delay control module that
 adjusts each of the plurality of independently adjustable optical delay devices to provide a coarse compensation delay for each of the plurality of modulated modal components, wherein the coarse compensation delay compensates a difference between a propagation time of the modulated modal component and a propagation time of a reference. 
 
 
     
     
       10. The optical receiver of  claim 9 , wherein the delay control module determines the coarse compensation delay for each of the plurality of modulated modal components using information from a data repository. 
     
     
       11. The optical receiver of  claim 10 , wherein the data repository comprises modal delay data and transmission length data. 
     
     
       12. The optical receiver of  claim 10 , wherein the data repository comprises transmission length data and modal delay data corresponding to a plurality of links of an optical network in which the optical receiver is installed, wherein the delay control module retrieves the length data and the modal delay data as a function of a link identifier of a multimode link. 
     
     
       13. The optical receiver of  claim 9 , wherein the delay control module
 determines a residual delay for each of the plurality of modulated modal components at one of the plurality of coherent optical detectors, 
 adjusts each of the plurality of adjustable optical delay devices to provide a residual delay compensation for each of the plurality of modulated modal components, and 
 updates a data repository using the residual delay. 
 
     
     
       14. The optical receiver of  claim 13 , wherein the delay control module updates a data repository using the residual delay. 
     
     
       15. The optical receiver of  claim 9 , wherein the delay control module adjusts each of the plurality of adjustable optical delay devices corresponding to a residual delay determined based on optimizing a cross-correlation function between a first digital stream recovered from a first modulated modal component and a second digital stream recovered from a second modulated modal component, wherein both modulated modal components were originally modulated with a same learning sequence. 
     
     
       16. The optical receiver of  claim 15 , wherein the delay control module updates a data repository using the residual delay. 
     
     
       17. The optical receiver of  claim 9 , wherein the signal processing device comprises an adjustable digital delay line that delays a digital signal component resulting from at least one of the plurality of modulated modal components, wherein the delay control module adjusts the adjustable digital delay line as a function of a residual delay determined for the at least one modulated modal component. 
     
     
       18. An optical receiver for multimode communications, comprising:
 a mode demultiplexer having an input and a plurality of output lines, the input being adapted to receive an optical signal having a plurality of modulated modal components, wherein each of the plurality of modulated modal components is coupled into one of the plurality of output lines; 
 a plurality of independently adjustable optical delay devices, wherein each of the independently adjustable optical delay devices is connected to one of the plurality of output lines; 
 a plurality of coherent optical detectors producing a plurality of electrical digital signals, wherein each of the plurality of coherent optical detectors is connected to one of the plurality of independently adjustable optical delay devices; 
 a signal processing device adapted to process the plurality of electrical digital signals; and 
 a delay control module that adjusts each of the plurality of independently adjustable optical delay devices to provide a coarse compensation delay for each of the plurality of modulated modal components, wherein the coarse compensation delay compensates a difference between a propagation time of the modulated modal component and a propagation time of a reference, and wherein at least one of the plurality of independently adjustable optical delay devices comprises a first delay device having a first plurality of optical fibers wherein each optical fiber has a different length and wherein one of the plurality of modulated modal components is substantially coupled into a selected one of the first plurality of optical fiber. 
 
     
     
       19. The optical receiver of  claim 18 , wherein at least one of the plurality of independently adjustable optical delay devices comprises a second delay device having a second plurality of optical fibers wherein each optical fiber has a different length and wherein one of the plurality of modulated modal components from the first plurality of optical fibers is substantially coupled into a selected one of the second plurality of optical fibers. 
     
     
       20. The optical receiver of  claim 19 , wherein the signal processing device comprises an adjustable digital delay line that delays a digital signal component resulting from at least one of the plurality of modulated modal components, wherein the delay control module adjusts the adjustable digital delay line as a function of a residual delay determined for the at least one modulated modal component.